This invention is a device and method that allows an objective measurement to be taken of the size and morphological pattern of the degradation of the image in scotopic vision, thanks to which it is possible to correct phenomena of glare and scotopic vision of light sources. It is a direct method of evaluation of these alterations or aberrations.
Nowadays, the development and improvement of refractive surgery techniques and their posterior widespread diffusion have increased interest among ophthalmologists in the control of visual quality.
The complaints of some patients concerning phenomena of glare and scotopic vision of light sources, with the prototypical or most common case being that caused by the headlights of oncoming vehicles while driving at night, have encouraged for some time now the need to know more about these sources and the introduction of different methods for their evaluation.
Glare tests have been used in the widest variety of illnesses such as keratoconus, corneal oedema, cataract and capsular opacification, as well as in the evaluation of the optic quality of intraocular lenses and refractive surgery.
The very complexity of the psychophysical perception of these phenomena of glare, the nature of the factors that cause them, as well as the wide variety of associated pathologies, make it difficult to evaluate them objectively using a standardized test method.
The typical manifestation is the appearance of aberrations in night vision of light sources, such as the headlights of an oncoming vehicle.
In general, any alteration of the parts of the eye that disperse light or multinodal patterns of focal distribution has the potential to produce aberrations of this type.
Previous studies on the results of refractive surgery with excimer laser consider that the component of dispersion of intracorneal light in the xe2x80x9cbeamxe2x80x9d is the main pathogenic factor of xe2x80x9cstarbustxe2x80x9d. In these patients, as is the case with other conditions which involve loss of transparency in the eyes, the reduction in contrast of the retinal image could be determined by the quantity of light dispersed.
Most of the tests available for the evaluation of this phenomenon study the effect of a glaring light source on contrast sensitivity tests or low contrast visual acuity optotypes.
Furthermore, the corneal dioptre power differential, between the treated and untreated area, is indicated as being responsible for the halo. Consequently, decentring and the small diameters of the ablated area, as well as the characteristics of the transition zone and the amplitude of pupillary excursions, are factors that favour this phenomenon. This focal multinodal distribution could also be responsible for the appearance of aberrations of scotopic vision of light sources both in patients who wear intraocular lenses, cataract post-surgery (in this case, obviously, the dioptre power differential would refer to the areas occupied and not occupied by the intraocular lens in the pupil), and in irregular processes on the cornea. To date, most authors evaluate these alterations using questionnaires or other subjective interpretations, and no sufficiently contrasted method of quantitative evaluation exists which has shown itself to have the high reproductive capacity of this phenomenon.
Although it is true that the previously mentioned methods of exploration are interesting and evaluate an important aspect of visual function, the correlation established between the results of these tests and the complaints of the patients is still being questioned. Moreover, considering the very fact that the perception of the distortion of night vision of light sources is responsible for discomfort and not the capacity to discriminate details or contrasts, it is considered necessary to introduce a new approach in the study of these visual anomalies.
Measurement of visual acuity or contrast using glare devices (BAT Mentor, Miller-Nadler glare test) shows that it is obvious that they provide an indirect evaluation of visual function which, although interesting, does not describe the halo or starbust which in fact leads to the complaint of those affected.
This device and method which allows an objective measurement to be taken of the size and morphological pattern of the degradation of image in scotopic vision consists of a front screen, with a completely dark background and which includes a central light source surrounded by a series of circular light spots laid out in a radial fashion.
The central light source that will be used as a reference for fixing has a diameter of 10 millimeters and luminance that varies according to the calibration carried out via software.
The grid which covers the central light source is composed of circular spots which are one millimeter in diameter and have the same light intensity as that selected for the central light source, laid out in 12 meridian lines. They are spaced according to the optimal configured progression, in which the first concentric circle of spots closest to the centre maintains a distance of 4 millimeters from the centre; the second, third and fourth concentric circles maintain a distance of 5 millimeters from the previous one, whereas from the fifth to the tenth the distance extends to 10 millimeters between each one.
The device described incorporates a remote control trigger, with a button, which, when pressed, produces a memorization of the target light spot. The spot is lit up and retained in memory for later processing. The device is in turn equipped with a port/connection that, via software, makes possible the compilation of this type of data, which, after processing by a computer, is printed, in the form of a map or area used for the calculation of the glare index.
Analytical test method of image degradation in scotopic vision
The light intensity of the spots is calibrated according to the patient and type of test to be carried out. The system is based on the control of a spot 1 mm in diameter around a central spot 10 mm in diameter. This spot circulates over the sectors in different modes of functioning according to their definition by the user from the software control:
mode 1: sequential, covering all the spots of the dial.
mode 2: sequential, covering only the part with greatest point intensity (closest to the central spot)
mode 3: random, covering all the spots of the dial.
mode 4: random, covering only the part with greatest point intensity (closest to the central spot)
mode 5: mode defined by the user, who selects the spots which he/she wants to be illuminated, these will go sequentially around the central spot.
With the interior in darkness and the patient at two meters from the test device, the patient is encouraged to fix one eye on the central light source, with the other eye covered until the end of the test.
If the process selected is sequential, the test begins with illumination of the spots of the test grid from the nearest to the central source of glare in centrifuge fashion towards the outermost, in the first and successive meridians.
If the process selected is random, the test begins with the illumination of the spots of the test grid with no predetermined order or rhythm, alternating between meridians and distances of approach to the center.
The time interval for the appearance of the test spots is variable and also programmed beforehand from the computer support. The patient is encouraged to press the button every time he can distinguish the presence of each of the light spots isolated from the central light focus.
Those non-discriminated light spots, which could be defined as hidden, border an area which is interpreted as occupied because of the luminous aberration and which serves as a basis for the calculation of the glare index (the index is obtained from the calculation of the percentage of the area covered by the halo, in relation to the test reference surface).
A fundamental condition for the performance of these determinations is the presence of scotopic environmental luminance that allows sufficient amplitude of night pupillary dilation. In previous studies, with all the lights in the room off and only the device on, suitable and stable environmental luminance was obtained. In these conditions, global illumination is below 0.17 lux.
We consider this method a direct method for the evaluation of these alterations or aberrations of scotopic vision. Application of this device and the method described reproduces in a laboratory the conditions of the prototype situation of night-time driving using an ad hoc light source of high intensity glare. In the measurements taken, the source luminance was calibrated at 458 Cd/m2, which is representative of the density of light sources that these visual aberrations usually produce. Other previously developed computerized devices use high-resolution monitors, but even these cannot provide such levels of luminance.